The present invention relates generally to the automatic control of a manufacturing process, such as the process of reducing the thickness of metal in a rolling mill, and particularly to automatically setting the exit gage of such metal close to a limiting value, though the invention is not limited to gage control and the control of rolling mills.
In reference to a rolling mill, as an example of the utility of the present invention, the thickness or gage of material leaving the mill is generally measured by a device located at the exit end of the mill. This device is part of an automatic gage control (AGC) system which functions to make corrections in the mill when the thickness of the metal leaving the mill deviates from a desired nominal gage value. There are times, however, when it is desirable to locate this nominal value closer to an upper or lower limit of the value. In the case of reducing the thickness of metal in a rolling mill, substantial savings in metal can be effected if the nominal is shifted down toward a lower limit but in a manner that does not allow variations in gage to go below the lower limit. By not permitting gage variations to go below the lower limit, a rolled product is provided with a minimum thickness value while simultaneously effecting the above savings in metal, overagainst the operation of the standard automatic gage control, which simply maintains the nominal within a bandwidth centered on the nominal.
Variations in the thickness of rolled metal are imparted to the metal in the rolling process. Such variations are due to many factors, including variations in the gage or hardness of the metal entering a mill, eccentricity in backup and work roll assemblies of the mill, variations in the tension of the metal between the stands of the mill, and other factors. The resulting gage of the metal exiting the mill is the sum of all of these variations, many of them cyclic and some due to long-term effects such as the growth in the diameter of the backup rolls, i.e., the temperature of mill rolls increases from room temperature, at the time of start-up of the rolling process, to a higher, steady state value after an interim period of time.